3. Gas turbine engines include one or more turbines for extracting energy from a stream of hot combustion gases that flow through an annular turbine flowpath. A typical turbine includes at least one stage of blades and one stage of vanes streamwisely spaced from the blades. Each stage of blades comprises multiple, circumferentially distributed blades, each radiating from a rotatable hub so that an airfoil portion of each blade spans across the flowpath. Each stage of vanes comprises multiple, circumferentially distributed nonrotatabale vanes each having airfoils that also span across the flowpath. It is common practice to cool the blades and vanes to improve their ability to endure extended exposure to the hot combustion gases. Typically, the employed coolant is relatively cool, pressurized air diverted from the engine compressor.
4. Turbine designers employ a variety of techniques, often concurrently, to cool the blades and vanes. Among these techniques is film cooling. The airfoil of a film cooled blade or vane includes an internal plenum and one or more rows of obliquely oriented, spanwisely distributed coolant supply holes, referred to as film holes. The film holes penetrate the walls of an airfoil to establish fluid flow communication between the plenum and the flowpath. During engine operation, the plenum receives coolant from the compressor and distributes it to the film holes. The coolant issues from the holes as a series of discrete jets. The oblique orientation of the film holes causes the coolant jets to enter the flowpath with a streamwise directional component, i.e. a component parallel to and in the same direction as the dominant flow direction of the combustion gases. Ideally, the jets spread out laterally, i.e. spanwisely, to form a laterally continuous, flowing coolant film that hugs or adheres to the flowpath exposed surface of the airfoil. It is common practice to use multiple, rows of film holes because the coolant film loses effectiveness as it flows along the airfoil surface.
5. Film cooling, despite its merits, can be challenging to execute in practice. The supply pressure of the coolant in the internal plenum must exceed the static pressure of the combustion gases flowing through the flowpath. Otherwise the quantity of coolant flowing through the film holes will prove inadequate to satisfactorily film cool the airfoil surfaces. At worst, the static pressure of the combustion gases may exceed the coolant supply pressure, resulting in ingestion of harmful combustion gases into the plenum by way of the film holes, a phenomenon known as backflow. The intense heat of the ingested combustion gases can quickly and irreparably damage a blade or vane subjected to backflow. However, the high coolant pressures required to guard against inadequate coolant flow and backflow can cause the coolant jets to penetrate into the flowpath rather than adhere to the surface of the airfoil. As a result, a zone of the airfoil surface immediately downstream of each hole becomes exposed to the combustion gases. Moreover, each of the highly cohesive coolant jets locally bifurcates the stream of combustion gases into a pair of minute, oppositely swirling vortices. The vertically flowing combustion gases enter the exposed zone immediately downstream of the coolant jets. Thus, the high pressure coolant jets not only leave part the airfoil surface exposed, but actually entrain the hot, damaging gases into the exposed zone. In addition, the cohesiveness of the jets impedes their ability to spread out laterally (i.e. in the spanwise direction) and coalesce into a spanwisely continuous film. As a result, strips of the airfoil surface spanwisely intermediate the film holes remain unprotected from the hot gases.
6. One way to encourage the coolant jets to adhere to the surface is to orient the film holes at a shallow angle relative to the surface. With the holes so oriented, the coolant jets will enter the flowpath in a direction more parallel than perpendicular to the surface. Unfortunately, installing shallow angle film holes is both expensive and time consuming. Moreover, such holes contribute little or nothing to the ability of the coolant to spread out laterally and coalesce into a continuous film.
7. A known film cooling scheme that helps to promote both lateral spreading and surface adherance of a coolant film relies on a class of film holes referred to as shaped holes. A shaped hole has a metering passage in series with a diffusing passage. The metering passage, which communicates directly with the internal coolant plenum, has a constant cross sectional area to regulate the quantity of coolant flowing through the hole. The diffusing passage has a cross sectional area that increases in the direction of coolant flow. The diffusing passage decelerates the coolant jet flowing therethrough and spreads each jet laterally to promote film adherance and lateral continuity. Although shaped holes can be beneficial, they are difficult and costly to produce. An example of a shaped hole is disclosed in U.S. Pat. No. 4,664,597.
8. What is needed is a cost effective film cooling scheme that encourages the cooling jets to spread out laterally across the surface of interest and to reliably adhere to the surface.